1. Field of the Invention
The present invention generally relates to a packet data mobile communication system using Frequency Division Multiple Access (FDMA), and in particular, to a method and apparatus for efficient open loop power control.
2. Description of the Related Art
Uplink multiple access schemes that have been recently used in mobile communication systems can be roughly divided into non-orthogonal multiple access scheme and orthogonal multiple access scheme. The non-orthogonal multiple access scheme as the name implies is a multiple access scheme, such as Code Division Multiple Access (CDMA), in which uplink signals transmitted from a plurality of mobile stations are not orthogonal to each other. The orthogonal multiple access scheme is also a multiple access scheme, such as Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA), in which uplink signals transmitted from a plurality of mobile stations are orthogonal to each other. A general packet data mobile communication system employs a combination of FDMA and TDMA as the orthogonal multiple access scheme. In other words, transmissions of multiple users are distinguished in terms of both frequency and time. In the following description, the combination of FDMA and TDMA will be referred to as FDMA. Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) are subsets of FDMA. In these FDMA techniques, a plurality of mobile stations transmit signals using different sub-carriers in order to allow the signals from the different mobile stations to be distinguished from one another.
A transmitter using Interleaved Frequency Division Multiple Access (IFDMA) that is an example of SC-FDMA will be described with reference to FIG. 1. FIG. 1 illustrates the structure of an IFDMA transmitter.
Although the IFDMA transmitter is implemented using Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) in FIG. 1, it can also be implemented differently. However, the implementation using FFT and IFFT as illustrated in FIG. 1 can facilitate easy change of IFDMA system parameters without complex hardware architecture.
OFDMA and IFDMA may have the following differences in terms of transmitter structure. In addition to an IFFT unit 106 used for multiple sub-carrier transmission, the IFDMA transmitter includes an FFT unit 104 located before IFFT unit 106. Transmission modulation (TX) symbols 100 are input to FFT unit 104 block by block. Signals output from FFT unit 104 are input to IFFT unit 106 at equal intervals, so that IFDMA transmission signal elements are transmitted in the frequency domain by sub-carriers of equal intervals. In this process, it is usual for the input/output size N of IFFT unit 106 to be greater than that of the input/output size M of FFT unit 104.
In the OFDMA transmitter, TX symbols 100 are directly input to IFFT unit 106 without passing through FFT unit 104 and are then transmitted by multiple sub-carriers, thereby generating a large Packet to Average Power Ratio (PAPR). However, in the IFMDA transmitter, even though TX symbols 100 are finally processed by IFFT unit 106 before being transmitted by multiple carriers, they are pre-processed by FFT unit 104 before being processed by IFFT unit 106. Due to the counterbalancing between FFT unit 104 and IFFT unit 106, the pre-processing of TX symbols 100 makes it possible to have an effect similar to that acquired when the output signals of IFFT unit 106 are transmitted by a single sub-carrier, thereby achieving a low PAPR. Finally, the outputs of IFFT unit 106 are converted to a serial stream by a Parallel-to-Serial Converter (PSC) 102. Before the serial stream is transmitted, a Cyclic Prefix (CP) is added to the serial stream as it is in the OFDMA system, thereby preventing interference between multi-path channel signal components.
FIG. 2 is a block diagram of a transmitter using a Localized Frequency Division Multiple Access (LFDMA) technique, which is similar to the IFDMA technique. The LFDMA technique guarantees the orthogonality between multiple access users and is based on single carrier transmission, thereby achieving a PAPR lower than that of the OFDMA technique.
As illustrated in FIGS. 1 and 2, the difference between LFDMA and IFDMA in view of the transmitter structure is that the outputs of an FFT unit 204 turn into inputs to an IFFT unit 206, which have sequential indices following the last index of the FFT unit 204. In the frequency domain, LFDMA signals occupy the band substituted by adjacent sub-carriers used when the outputs of FFT unit 204 are mapped to the inputs of IFFT unit 206. In other words, IFDMA signals occupy sub-carrier bands (sub-bands) distributed at equal intervals and the LFDMA signals occupy the sub-band constituted by adjacent sub-carriers.
In a general uplink mobile communication system, a base station can support high system capacity with limited radio resources through channel selective scheduling. “Uplink” means transmission from a mobile station to the base station. “Channel selective scheduling” is a technique for allocating a time interval or a frequency interval having a good channel condition to a channel that changes on a time axis or a frequency axis so as to improve system capacity.
FIG. 3 is a scheduling flow diagram on the time axis.
Referring to FIG. 3, a Mobile Station (MS) 302 transmits a pilot signal to a Base Station (BS) 301 in step 303 to be scheduled for data transmission. BS 301 recognizes the channel status of MS 302 based on the received pilot and determines whether to schedule data transmission. When BS 301 determines to schedule data transmission, it determines an appropriate modulation method and a coding rate. In step 304, MS 302 reports its status to BS 301. The status means a buffer status or a power status that the MS desires to transmit. The buffer status information may be the amount of packet data or a service priority of packet data, and the power status information may be the possible amount of transmission power. BS 301 performs scheduling based on the status information and pilot information in step 305. When MS 302 is scheduled, BS 301 transmits a scheduling grant for data transmission to MS 302 in step 306. MS 302 having received the scheduling grant transmits packet data to BS 301 in step 307.
In the aforementioned general scheduling process, an MS may transmit power information to a BS according to a transmission power setting algorithm or may not transmit the power information. In other words, a BS can know the allowable maximum data rate only when the MS transmits power headroom information to the BS in a system that uses closed loop power control; but in a system using open loop power control, the BS can recognize the allowable maximum data rate only with a Signal-to-Interference Ratio (SIR) of a pilot signal received from the MS without reception of power information from the MS.
In a system based on FDMA of orthogonal multiple access type, an MS in a cell is granted orthogonal resources and thus MSs in the cell do not interfere with one another, thereby reducing the need for closed loop power control that is essential for non-orthogonal multiple access. Moreover, closed loop power control requires feedback information for power control. For these reasons, a system that desires to adopt FDMA prefers open loop power control over closed loop power control to control the transmission power of an MS in consideration of signaling overhead.
A method for open loop power control will now be described.
A representative method for open loop power control is as Equation (1):PTX=Lpilot+IBTS+SIRTARGET  (1),
where PTX is a transmission power level (dBm) of a Dedicated Physical Channel (DPCH), Lpilot is a pathloss (dB) that is estimated using the measurement of a pilot channel and the signaled transmission power of a pilot channel, IBTS is an interference level (dB) a receiver of the BS experiences, and SIRTARGET is a target SIR (dB) for maintaining the transmission quality of each MS. SIRTARGET may be signaled separately for each MS or collectively for MSs.
When data is transmitted after the transmission power is set as described above, the receiver of the BS can receive a target SIR. However, a fading channel is not considered in Equation (1) and thus the actual reception SIR may not perfectly match with the target SIR due to the fading channel.
The relationship between a target SIR and scheduling will now be described.
Packet scheduling is intended to efficiently grant radio resources of a cell based on the buffer status and power status of an MS. With support for open loop power control, the power status of the MS is determined by the actual reception SIR of a pilot channel. For example, when a reception SIR of a pilot channel, which is transmitted from the MS, is 3 dB, a BS grants [16-Quadrature Amplitude Modulation (QAM), ⅓] as a Modulation Coding Selection (MCS) level for satisfying transmission quality to the MS. For a reception SIR of 0 dB, the BS grants [Quadrature Phase Shift Keying (QPSK), ⅓], which is lower than [16-Quadrature Amplitude Modulation (QAM), ⅓], to the MS. As a result, the MS having a high target SIR also has a high reception SIR and thus a BS scheduler is likely to grant more radio resources to the MS. However, when the highest target SIR is set for all MSs, transmission power increases, thus increasing interference with other cells in the uplink direction. On the other hand, when a low target SIR is set for all MSs, the MS has to transmit much data and may not be properly scheduled even with sufficient transmission power. For these reasons, in the application of open loop power control to packet transmission, it is more desirable to set different target SIRs separately for MSs based on individual conditions of the MSs than to set the same target SIR collectively for the MSs. However, in case of different target SIRs for MSs, when a target SIR is signaled to each MS using upper signaling, frequent signaling is not possible due to signaling overhead, thus making frequent setting of a target SIR difficult. Moreover, when an MS moves, accurate setting of a target SIR is not possible. Therefore, there is a need for a new method for open loop power control in order to efficiently change a target SIR without upper signaling overhead when an MS located adjacent to a BS initially transmits a packet.